1. Field of the Invention
The present invention is directed to down-draft fixed bed gasifiers for converting biomass material such as wood and processed sewage sludge to a low BTU combustible gas. More particularly the invention is directed to such a gasifier having a cleaning system for processing the gas produced from the gasifier.
2. Background of the Invention
The expense and potential shortage of petroleum-based fuels makes the use of a biomass fuel an attractive alternative. At the same time, biomass materials such as wood waste and other wastes such as sewage and/or industrial sludge are in abundant supply in most parts of the world. Indeed, supplies of such materials are so extensive that disposal of wood waste and sludge has become a problem. It would be advantageous to efficiently dispose of such materials and, at the same time, obtain useful fuel products from them.
Gasifiers which convert wood products, garbage and biomass materials to combustible gases are disclosed, for example, in U.S. Pat. Nos. 4,659,340; 4,583,992; and 4,348,211. However, many currently known gasifiers lack efficiency because it is difficult to achieve acceptable horizontal temperature control (i.e., stratified horizontal zones of constant temperature) within the gasifiers. Moreover, many known gasifiers may produce combustion by-products which have a high ash and particulate matter content. Accordingly, there is a need for an apparatus which will efficiently convert biomass materials (i.e., wood scrap and sewage sludge) to useful fuel products.